1. Field of the Invention
This invention relates in general to magnetic transducers, and more particularly to an apparatus for providing transverse magnetic bias proximate to a pole tip.
2. Description of Related Art
The first disk drive was introduced in the 1950s and included 50 magnetic disks that were 24-inch in diameter rotating at 1200 RPM (rotations per minute). There has been huge progress in the field of hard disk drive (HDD) technology in almost 50 years since the introduction of the first disk drive. Moreover, the rate of this progress is increasing year after year. Such success has made hard disk storage by far the most important member of the storage hierarchy in modern computers.
The most important customer attributes of disk storage are the cost per megabyte, data rate, and access time. In order to obtain the relatively low cost of hard disk storage compared to solid state memory, the customer must accept the less desirable features of this technology, which include a relatively slow response, high power consumption, noise, and the poorer reliability attributes associated with any mechanical system. On the other hand, disk storage has always been nonvolatile; i.e., no power is required to preserve the data, an attribute which in semiconductor devices often requires compromises in processing complexity, power-supply requirements, writing data rate, or cost.
Improvements in areal density have been the chief driving force behind the historic improvement in hard disk storage cost. In fact, the areal density of magnetic disk drives continue to increase, with currently commercially disk drives available with areal densities over 100 billion bits per square inch. While nature allows us to scale down the size of each bit of information, it does not allow scaling to happen forever. Furthermore, while these difficulties have been associated with hard disk drives, similar conclusions would apply to magnetic tape and other magnetic technologies.
Today, as the magnetic particles that make up recorded data on a hard disk drive become ever smaller, technical difficulties in writing and reading such small bits occur. Further, as areal density increases, the requirements put on head designs will change.
The most important customer attributes of disk storage are the cost per megabyte, data rate, and access time. However, improvements in areal density have been the chief driving force behind the historic improvement in hard disk storage cost. However, the present problems encountered in increasing areal density are more fundamental than problems encountered previously. These problems include the thermodynamics of the energy stored in a magnetic bit, difficulties with head-to-disk spacings that are only an order of magnitude larger than an atomic diameter, and the intrinsic switching speeds of magnetic materials.
One area that is increasing steadily is the data transfer rate. The signal frequency of the current state of the hard disk drives continues to rise. Basic scaling for magnetic recording is the same as the scaling of any three-dimensional magnetic field solution: If the magnetic properties of the materials are constant, the field configuration and magnitudes remain unchanged even if all dimensions are scaled by the factor s, so long as any electrical currents are also scaled by s. In the case of magnetic recording, there is the secondary question of how to scale the velocity or data rate to keep the dynamic effects mathematically unchanged. Unfortunately, there is no simple choice for scaling time that leaves both induced currents and electromagnetic wave propagation unchanged. Instead, surface velocity between the head and disk is usually kept unchanged. This is closer to engineering reality than other choices. It means that induced eddy currents and inductive signal voltages become smaller as the scaling proceeds downward in size.
Therefore, if we wish to increase the linear density (that is, bits per inch of track) by 2, the track density by 2, and the areal density by 4, we simply scale all of the dimensions by half; leave the velocity the same, and double the data rate. If the materials have the same properties in this new size and frequency range, everything works as it did before.
That constitutes the first-order scaling. In real life, there are a number of reasons why this simple scaling is never followed completely. For magnetoresistive (MR) heads, the scaling laws are more complex, but tend to favor MR increasingly over inductive heads as size is decreased. The last reason, which will ultimately cause very fundamental problems, is that the materials are not unchanged under the scaling process; we are reaching physical dimensions and switching times in the head and media at which electrical and magnetic properties are different than they were at lower speeds and at macroscopic sizes.
In today's recording density, in particular with high track-per-inch, the track-width of the write-head (P2B) is getting ever smaller. In the state-of-the-art server-drive, P2B is already approaching the neighborhood of 0.25 um. For desktop and laptop drives, P2B is even smaller because of the higher areal density required of those applications. On the other hand, the data-rate is getting higher. In the case of server-drives, the data-rate is approaching 1 Gb/sec.
The problems associated with the increased data rates described above involve the switching the magnetization of the pole-tip. Switching of the magnetization of the pole-tip at such a high data-rate, especially with the pole-tip dimensions being comparable to or even smaller than those of a single magnetic domain, is becoming a serious challenge. There has been experimental evidence that the pole-tip's magnetization is switching much slower than that of the much wider yoke in the back. In fact, there are even indications that for a very narrow pole-tip and at high enough frequencies, the pole-tip is no longer acting as a soft-magnet but almost as a tiny single-domain hard-magnet, with its magnetization being switched back and forth to do the high data-rate writing.
The sluggishness of the pole-tip switching action is partially due to the significant shape- and stress-induced longitudinal anisotropy, which is caused by its small dimension and elongated shape. Accordingly, the problem of enhancing the pole-tip magnetization switching in the presence of significant longitudinal anisotropy (be it shape- and stress-induced) needs to be addressed.
Recently, the switching time of a single-domain particle with uniaxial anisotropy and collinear applied magnetic field has been addressed. See, for example, J. C. Mallinson, IEEE Trans. Magn., Vol-36, pp. 1976–1981, July, 2000, which is incorporated herein by reference. One particular observation is that the time to rotate the magnetization to a 90-degree polar angle (from near-zero) is always greater than the time to proceed from 90-degrees to near 180-degrees in the presence of an applied field collinear with the easy axis. This is because of the dependence of the effective anisotropy field Hk on the polar angle theta (Θ), in which Hk is proportional to cos(Θ). In other words, it is because the effective Hk opposes the switching in the first 90-degrees, i.e., 0° to 90°, while it helps the switching in the second 90°, i.e., 90° to 180°, of the total switching process. Also, the magnitude of the effective Hk is stronger at 0° than at 90°. However, the single-domain particle model with uniaxial anisotropy and the collinear applied field is a rather simple and ideal case.
In contrast, with regard to the problem of enhancing the pole-tip magnetization switching, the elongated shape (currently about 0.25 um wide, 1.2 um high, and 2 um long) almost guarantees that the shape-anisotropy is perpendicular to the ABS (air-bearing-surface), longitudinal to the pole-tip. In addition, the driving flux transduced by the yoke further back in the writer is injected into the pole-tip region at its back-end, almost collinear to the shape-anisotropy axis.
The problem of slow magnetic-switching of the pole-tip is usually tackled by introducing an overshoot in the write-current during the current switch. This current-overshoot provides an extra driving field to overcome the initial hurdle in switching the pole-tip magnetization. However, the extra driving-field given by current-overshoot has side effects such as creating excessive erase-band, extra adjacent-track-interference (ATI), and extra protrusion.
It can be seen that there is a need for an apparatus for providing transverse magnetic bias proximate to a pole tip to speed up the switching time of the pole-tip during the writing operation.